A known ultrafiltration apparatus, described in British Pat. No. GB 1 468 928 (Union Carbide Corp.), comprises a plurality of parallel tubes arranged so that a liquid to be treated passes under pressure axially through the tubes. The tubes are porous and permeable, being composed of bonded carbon particles and an essentially continuous coating of metal oxide particles on the inside surface of the tubes. In operation of the apparatus some of the liquid and low molecular weight dissolved phases pass through the walls of the tubes, while most of the higher molecular weight dissolved phases and any particulate matter are carried through the tubes in the liquid concentrate.
Such an apparatus may be used to separate water from an oil/water or oil/water/dirt emulsion. Equally it may be used to remove water from a sludge of particulate matter and water. For example it might be used to dewater a sludge containing low-level radioactive material, before immobilizing the sludge by the addition of cement. In such a method it may be advantageous to monitor the degree of dewatering so as to ensure that the solids concentration of the sludge reaches an appropriate final value, and such monitoring must be able to cope with wide variations in the nature of the initial sludge, for example its chemical composition, solids concentration or particle size.